Project Summary/Abstract Syphilis is increasing in the United States (U.S.) and Baltimore has one of the most severe epidemics among Black gay, bisexual and men who have sex with men (MSM). The racial disparities and number of new syphilis infections and reinfections demonstrate that our current prevention strategies are not effective. The proposed study will provide a new understanding of the epidemiology and transmission dynamics of syphilis among Black MSM including HIV infected and uninfected MSM for urban areas in the U.S. in order to identify ways to alter and strengthen local health department practices. We will explicitly address limitations in current practice and previous research on key aspects of the syphilis prevention-care continuum including: syphilis case finding, access to and treatment completion, treatment follow-up and the likelihood of reinfection and co- infection with HIV and other STIs over time. Our approach will include a cohort study of Black MSM, respondent driven-sampling and a focus on places where sexual networks are formed such as sex partner meeting venues (e.g. clubs, bars, internet sites, parks) and their context, as venues have been shown to be an effective method of accessing hard-to-reach populations at risk for syphilis. Our Specific Aims are to: (1) Determine sex partner meeting venues and their characteristics (e.g. type, venue network centrality) as well as social and sexual network individual risk behaviors (e.g. sex without a condom, new partners, substance use) associated with new syphilis incidence and repeat syphilis over time; (2) Determine individual (e.g. health care access and usage patterns, insurance status, distance to clinic, substance use), social network (e.g. support, risk network) and systems level (e.g. availability of health care services, cultural competency factors. perceived stigma) healthcare-related factors associated with treatment completion and engagement in follow-up for syphilis over time; and (3) Examine the impact of clinical factors (e.g. syphilis stage, reinfection, baseline nontreponemal titers, HIV and PrEP status) on changes in non- treponemal antibody titers over time following treatment. Our study will be cooperatively designed and conducted by the Baltimore City Health Department (BCHD), Johns Hopkins University (JHU) and CDC to inform public health targeted control activities for syphilis and co- morbid syphilis/HIV infection. The BCHD and JHU have a long-standing, successful partnership in the translation of evidence to public health practice. In addition, our expert team has extensive experience and demonstrated past performance in recruitment and retention of MSM in longitudinal studies, and in the collection of sociodemographic, epidemiologic, sexual and social network data and biologic specimens for research purposes. In addition, we bring a rich track record of syphilis clinical care and research, sex partner meeting place research and translation of evidence to public health practice.